Metal-organic frameworks (MOFs) are materials obtained by the assembly of small inorganic clusters and organic linkers to form one-, two-, or three-dimensional (3D) crystalline networks. [1] The synthesis of 3D porous MOFs with large surface area and pore volume and chemical stability, coupled with the ability to tailor pore sizes and functionality during the synthesis step, makes these materials promising in hydrogen storage, [2] separation, sequestration of carbon dioxide [5] or harmful gases, [6] and catalysis. 7] When transition metal ions are incorporated in the MOF structures, these materials show interesting optical, ionic conductivity, [9] and magnetic properties that, combined with their exceptional porous characteristics, provide the opportunity to develop new low-density multifunctional materials.In this Communication, we present the facile and fast microwave synthesis of MIL-47, one of the most studied MOF materials, and 6 new vanadium MOFs obtained under similar conditions using different organic building blocks. We show that these different organic building blocks affect the interactions between V ions, and, consequently, the magnetic properties of the MOF. Furthermore, we studied the effect on the magnetic properties when guest molecules were removed from the MIL-47 pores. It was suggested that changes in the magnetic properties upon guest-molecule desorption are due to a change in the vanadium oxidation state from V 3þ to V 4þ . In this Communication, we will show that the changes in the material magnetic properties are due to the distortion of the octahedral vanadium sites and the consequent splitting of the previously degenerate three vanadium t 2g orbitals into a lower-energy orbital and two higher-energy orbitals, a phenomenon similar to the energy splitting in the Jahn-Teller effect. [15] Under these conditions, the two d electrons of V 3þ occupy only the lower-energy orbital and the total spin angular momentum becomes zero, thus dramatically changing the material's magnetic properties.MIL-47 consists of corner-sharing vanadium (III) oxide octahedra chains connected by terephthalate linkers (Figure ). The framework, composed of organic and inorganic building blocks, generates 1D cavities that are filled by terephthalic acid molecules in the as-synthesized material (MIL-47as). MIL-47as was readily synthesized at 200 8C with a new procedure in which a vial containing terephthalic acid, vanadium trichloride, and deionized water in a 4:1:100 molar ratio was irradiated with microwaves (200W) for 10 min. The typical yield was %25% based on the initial vanadium content. This microwave synthesis is a significant improvement over the hydrothermal method reported in literature, which takes four days to run and results in a lower yield. The material was filtered, dried overnight, and characterized. The MIL-47as X-ray diffraction (XRD) pattern matches well with previously published results (see the Figure of the Supporting Information). Several other vanadium MOF (V-MOF) materials were synthesized under similar conditions using VCl 3 as the vanadium source and different dicarboxylic ligands as organic building blocks (Figure ). These materials are named V-MOF-2 through V-MOF-7 (with the V-MOF-1 designation reserved for the original MIL-47). Table lists the new V-MOFs and the dicarboxylic ligands used for their synthesis, which are also shown in Figure . These results highlight the great potential of microwave synthesis for preparing new MOF materials, in accord with the pioneering studies of Jhung et al. [16] The microwave synthesis did not produce single crystals for structural analysis but we were able to determine the unit-cell lattice parameters for the newly synthesized MOFs using powder XRD (Table and ). It is important to note that the shortest unit-cell lattice parameter, corresponding to the direction along the vanadium oxide chains, is about 7 A ˚for all the materials studied. For MIL-47as, the terephthalic acid molecules in the pores are highly disordered and previous authors could not refine their position even with single-crystal data. Despite their being in a disordered state, the guest molecules contribute significantly to V-MOF powder XRD patterns (Figure ). We used isoreticular model structures with one of the building blocks kept constant (vanadium oxide communications [ Ã ] Dr.